During fiscal year 2011, we accomplished the following: 1. We identified homologous recombinants amongst greater than 200 embryonic stem (ES) cell clones using Southern blotting and PCR assays. These ES cells were generated using targeting vectors that introduced LacO or TetO binding-sites in the 1) VH locus, 2) 3 regulatory region, 3) DH locus, and 4) within the intronic enhancer, E. The identified clones are being used for embryo reconstitution to generate targeted mice. 2. We developed stable cell lines in mouse embryo fibroblasts (MEFs) using two manipulated bacterial artificial chromosomes (BACs). Both BACs are derived from the VH region of the murine IgH locus but differ in the number of naturally occurring L1 retrotransposons. We will use these cell lines to determine if L1 retrotransposons can initiate heterochromatinization and drive associated sequences to the nuclear periphery.